code: def test_demo_plot_roi_output_file(tmp_path):
      """Tests plot_roi file saving capabilities."""
      filename = tmp_path / "test.png"
      with open(filename, "wb") as fp:
          out = demo_plot_roi(output_file=fp)
      assert out is None
  
file_path: nilearn/nilearn/plotting/tests/test_img_plotting/test_plot_roi.py
imports: import matplotlib.pyplot
  import numpy
  import pytest
  from nibabel import Nifti1Image
  from nilearn.conftest import _affine_mni
  from nilearn.image.resampling import coord_transform
  from nilearn.plotting import plot_roi
label_desc: Use image node_id 4 for calling a global function with example usage: test_demo_plot_roi_output_file(tmp_path) without return types